The present invention relates to a method and a system for controlling the creep behavior of a vehicle equipped with an automated clutch.
Automated clutches are being increasingly used not only because of the added convenience they provide, but also because of the possible reduction in wear in motor vehicles.
FIG. 2 shows a schematic diagram of a power train of a motor vehicle equipped with an automated clutch as an example. The power train includes an internal combustion engine 2, a clutch 4, and a transmission 6, which is connected to driven wheels (not shown) via a drive shaft 8. Transmission 6 is an automated manual shift transmission, for example, a taper disk belt transmission having a continuously variable transmission ratio, or a conventional automatic transmission having planetary gears. A shifting device 9, which is controllable from a selector device 10 using a selector lever 12 via a control unit 14 in a known manner, is used to actuate or shift transmission 6. It is understood that the selector device may also have a different design, for example, as a conventional shift stick (H gate) or as a lever having jog positions for shifting up and down. Clutch 4 is a friction disk clutch, for example, of a design which is known per se, having an actuating device 16, which may have a hydraulic, electric, electrohydraulic, or other known design.
The sensors contained in the power train, such as a pressure sensor 18 for detecting the intake pressure of engine 2, a rotational speed sensor 20 for detecting rotational speed nM of the engine crankshaft, a sensor 22 for detecting position α of an accelerator pedal 24, a sensor 26 for detecting the position of selector lever 12, and an additional rotational speed sensor 28 for detecting the rotational speed of drive shaft 8, are connected to the inputs of control unit 14.
Control unit 14, having a microprocessor with respective memories 29, contains characteristic maps and programs in an essentially known manner, which control actuators, such as a load adjustment element 30 for adjusting the load of engine 2, actuating device 16, clutch 4, and shifting device 9 of transmission 6, and other consumers 31 driven directly or indirectly by the engine such as a generator, a pump, or a heating element, etc. The individual actuators may be designed in such a way that their position is immediately known in control unit 14, for example, as stepping motors, or additional position transducers, such as position transducer 32 for detecting a parameter relevant to position sK of clutch 4, may be provided.
A brake pedal 34 is connected, via a hydraulic line 35, to a braking pressure control unit 36, which is connected to vehicle brakes 38 via additional hydraulic lines. An additional electronic control unit 40, which is connected to control unit 14 via a data line 42, is provided for controlling brake pressure control unit 36. The pressure in hydraulic line 35 generated by the actuation of brake pedal 34 is detected by a pressure sensor 44 connected to control unit 14. Control unit 40 controls the brakes, for example, in an essentially known manner, in such a way that locking of a wheel is prevented (ABS system) and/or that the vehicle does not skid unintentionally (vehicle stability system). For this purpose, additional sensors (not shown) are provided, whose signals are analyzed in control unit 40, possibly together with signals delivered by control unit 14, so that the individual vehicle brakes and, if present, load adjustment element 30, may be triggered as needed. The distribution of hardware and software between devices 14 and 34, as well as the connections of the sensors and actuators are adapted to the particular conditions.
The design and function of the above-described system are essentially known and will therefore not be described in detail. Depending on the driver's intent communicated via accelerator pedal 24 and the intent of selecting a driving program or a driving direction communicated via selector lever 12, load adjustment element 30, actuating device 16, and shifting device 9 are actuated in a mutually coordinated manner as a function of the signals delivered by the sensors in such a way that comfortable and/or economical driving results.
A characteristic curve, for example, which determines a setpoint position of clutch 4 set by actuating device 16 as a function of the torque to be transmitted by clutch 4 is stored in the memory of control unit 14 to actuate clutch 4. For reasons of control quality, clutch wear, and power consumption of the actuating device, the clutch torque to be transmitted at a given time should not exceed the absolutely necessary value. The required torque to be transmitted results from the driver's intent, i.e., the position of accelerator pedal 24 and, for example, from the load on internal combustion engine 2, detected by sensor 18, and possibly from additional operating parameters such as the rotational speed of engine 2, etc.
The characteristic curve stored in control unit 14, which provides the desired path of the clutch's final control element moved by actuating device 16 as a function of the calculated torque to be transmitted, has a decisive influence on comfortable start and comfortable shifting. The characteristic curve changes in the short-term due to temperature changes, for example, and in the long-term over the lifetime of the clutch due to wear, for example. It is, therefore, constantly updated, i.e., readjusted, according to a wide variety of strategies, to the prevailing operating conditions.
One important function which is made possible by an automated clutch is the vehicle creep, which makes it possible for the vehicle to move slowly when a forward or reverse gear is selected and the engine is running, without actuating the accelerator pedal. The driver is thus able to maneuver the vehicle more easily by operating only the brake pedal to stop the creeping vehicle. In creeping, the clutch is controlled in general in such a way that it is engaged as long as it transmits a certain creep torque, for example, 10 Nm. This creep torque is controlled, for example, by control unit 14 slowly engaging clutch 4 using actuating device 16 with the transmission in gear and the gas pedal not actuated, while engine 2 is controlled via load adjustment element 30 of engine 2 at, as far as possible, a constant speed of engine 2 in such a way that it generates the predefined torque on the clutch. It is understood that the power consumption of any additional consumers driven by the engine is taken into consideration.
One peculiar feature of the known creep behavior of vehicles is that the driver is not able to directly influence the creep, which impairs comfort and requires a high degree of skill from the driver when, for example, he must maneuver on a sloping roadway.